The present invention relates to a time slot management method as defined in the preamble of claim 1, a related network terminator as defined in the preamble of claim 7, and a related line termination as defined in the preamble of claim 8.
Such a method and related device are already known in the art, e.g. from “ITU-T Recommendation G.983.1 Broadband optical access systems based on passive Optical Networks (PON)”.
Therein, the method and related devices are described in more specifically 155/155 Mbit/sec and 622 (downstream)/155 Mbit/sec systems wherein upon detection of its identity in a received downstream grant, a network terminator can use a 155 Mbit/s upstream timeslot to send upstream an upstream data packet.
In this G.983.1 Recommendation, more particular at paragraph 8.3.4 and paragraph 8.3.5 the upstream interface and the transport specific TC functions is described. Herein, it is indeed described that the upstream interface comprises the presence of overhead bytes that are added in front of each upstream Asynchronous Transfer Mode (ATM) cell. An upstream timeslot comprises 56 bytes i.e. 3 overhead bytes in front of 53 payload bytes i.e. an ATM cell. In Table 6/G.983.1 the overhead bytes are described. This table describes a guard time, a preamble and a delimiter. The guard time is defined in order to provide enough distance between two consecutive cells to avoid collisions. The preamble is defined in order to enable a line terminator, upon reception of an upstream data packet, to extract the phase of the arriving cell relative to the local timing of the line terminator and/or to acquire bit synchronisation and amplitude recovery. The delimiter is defined as a unique pattern that indicates the start of the ATM cell that can be used to perform byte synchronisation.
A disadvantage of the above described method to share upstream bandwidth is that in the event of an upgrade of a network terminator and line terminator towards a higher upstream transmission rate with e.g. a bitrate that increases above 155 Mbps, the upstream overhead such as the above described 3 bytes, becomes too short for the required guard time and transmitter setting and receiver setting and indication of the start of the payload. Indeed, the relative duration of the upstream overhead bytes decreases with increasing bitrate. Within this relatively shortened period of time it gets very complicated to have a sufficient guard time, to set all transmitter settings, to set all receiver settings and to indicate the start of the payload.
Physical layer overhead in currently standardised solutions is not adequate at gigabit rates. Just increasing the length means that the overhead to payload ratio for all the information packets (Physical Layer Operations, Administration and Maintenance (PLOAM) messages, ATM cells, modem buffer status reporting) will become inadequate.